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The problem with the Standard Approach

The survey parameters in Table 3 seem reasonable and well within the usual range, but there is a problem:
 
Table 3: Parameters of the standard design: dr is the receiver interval, ds is the source interval, drl is the receiver line interval, dsl is the source line interval, nchl is the number of channels per line and nrl is the number of receiver lines in the recording patch. Units are meters.
dr ds drl dsl nchl nrl
20 20 400 500 150 8

With an orthogonal geometry, there is a clear limit to the minimum offset that we can sample in all the bins as shown in Figure [*]. The shortest offset in the central bin is approximately 700 m, which is too large compared to the depth of the shallowest part of the target reflector. With the basic assumption of flat layers and constant velocity, this means that the central bins will not have offsets short enough to image the shallow part of the target reflector. Since this is the main target, I can't afford to compromise its image. From Figure [*] it should be clear that in order to decrease the maximum minimum offset we need to decrease the source line interval, the receiver line interval or both. In this particular case, we can simply halve them so that the maximum minimum offset is now just over 300 m. Obviously, halving the dsl doubles the number of required shots which in turn may double the cost of the survey. Furthermore, if the number of receiver lines is kept constant (which may be necessary if not enough equipment is available) the aspect ratio will also double, making the survey highly azimuthal, which may not be what we want. A possible solution is to use rectangular bins with the source interval equal to twice the receiver interval. This keeps both the number of shots and the aspect ratio constant, but may be an undesirable solution if significant dips are present in the strike direction (I am assuming that the source lines are in the strike direction).

 
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Figure 5
Recording box. Receivers are along the horizontal lines and sources along the vertical lines. The minimum possible offset for the bins in the middle of the box is of the order of 700 m, much too large to image the target reflector at 300 m depth.
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next up previous print clean
Next: The Proposed Approach Up: Alvarez: 3-D survey design Previous: Standard 3-D Survey Design
Stanford Exploration Project
7/8/2003